Spinels represent a class of ceramics represented by the structure M.sup.1 (M.sup.2).sub.2 O.sub.4 where M.sup.1 is a divalent metal cation and M.sup.2 is a trivalent metal cation and the oxygen anions form a cubic lattice. The aluminate spinels, having the general formula M.sup.1 Al.sub.2 O.sub.4, where M.sup.1 is a divalent metal, typically form from solid state reactions of the constituent oxides at approximately 1000.degree. C. The mineral Spinel, specifically MgAl.sub.2 O.sub.4, is a naturally-occurring macrocrystalline mineral.
Magnesium aluminate spinels as layered crystallites are described in U.S. Pat. No. 4,400,431.
Oriented magnetic ceramic fibers including nickel ferrite spinels are disclosed in U.S. Pat. No. 2,968,622.
Techniques for preparing spinels are disclosed in U.S. Pat. Nos. 4,394,455 and 4,532,121. For a general reference on spinels see E. Ryshkewitch, "Oxide Ceramics", Academic Press, New York, 1960, Chapter 3, and British Pat. No. 1,402,544.
Aluminate spinels have a variety of uses such as kilnware, refractory structures requiring molten metal slag resistance, ceramic protection tubes and fibers. A method for making polycrystalline refractory fibers, including a magnesium spinel fiber, is disclosed in U.S. Pat. No. 3,322,865. Spinels are also useful for their magnetic properties (e.g., the ferrites), for colorizers, stains, and abrasives. Normal fine grained spinels have grain sizes of 5-10 micrometers.